(Sorry for the uninformative title, but I had to work within the character limit.)

### Please describe the problem.

`git-annex metadata` does nothing on Windows if invoked while `git-annex addurl` is in progress on other files.

### What steps will reproduce the problem?

On Windows (but not on Linux or macOS, where everything works fine):

- Start `git-annex addurl` in batch mode
- Feed it two or more URLs
- After reading the completion message for a URL from addurl's stdout, but before reading the remaining output, run `git-annex metadata` in batch mode and try to set the metadata for the file that was just downloaded.
- `git-annex metadata` will output an empty line (i.e., just CR LF), and if nothing further is fed to it, it will exit successfully without printing anything else on stdout or stderr.
- Querying the file's metadata normally after `git-annex addurl` exits will show that no metadata was set for the file.

The Python script at <https://github.com/jwodder/git-annex-bug-20221024/blob/master/mvce.py> (Python 3.8+ required) will run the above steps and show the output from `git-annex metadata`.  A sample run under GitHub Actions can be seen at <https://github.com/jwodder/git-annex-bug-20221024/actions/runs/3322463020/jobs/5491516209>; note the following section of the output under "Run script":

```
16:04:04 [DEBUG   ] __main__: Opening pipe to: git-annex metadata --batch --json --json-error-messages
16:04:04 [DEBUG   ] __main__: Input to metadata: b'{"file": "programming/gameboy.pdf", "fields": {"title": ["GameBoy Programming Manual"]}}\n'
16:04:04 [DEBUG   ] __main__: r.returncode=0
16:04:04 [DEBUG   ] __main__: r.stdout=b'\r\n'
16:04:04 [DEBUG   ] __main__: r.stderr=b''
```

This problem does not always occur, but it seems to occur most of the time.  Using `git-annex registerurl` in place of `git-annex metadata` works fine.

### What version of git-annex are you using? On what operating system?

git-annex 10.20221003, provided by datalad/git-annex, on Microsoft Windows Server 2022 

### Please provide any additional information below.

This affects a hobby project of mine – "gamdam", implemented in [Python](https://github.com/jwodder/gamdam) and [Rust](https://github.com/jwodder/gamdam-rust) — that interacts with git-annex.

[[!meta author=jwodder]]

> [[fixed|done]], see my comments --[[Joey]]
